A two‐layer fluid which is inviscid and which has no conduction is considered. Once rotation is included in the perturbation equations, cubic equation for the square of the wave speeds of two‐dimensional waves results. Two of the roots can be closely approximated in rather simple analytic form. These roots are generally more unstable than the nonrotating waves. The root which is not degenerate as rotation approaches zero owes its greater instability under the presence of rotation to the fact that the fluid interface is tilted with respect to the gravity vector and the stabilizing role of the stratification is thereby reduced.